1. Field of the Invention
The present invention relates generally to a planer lightwave circuit, and in particular, to an optical power splitter.
2. Description of the Related Art
In general, a planar lightwave circuit includes a semiconductor substrate, a core, which is layered on the semiconductor substrate, for propagating inputted optical signals using total internal reflection, and a clad encompassing the core. An optical circuit using such waveguide would also include optical power splitters/combiners for splitting or combining power of optical signal, and wavelength division multiplexers/demultiplexers for multiplexing or demultiplexing channels that compose optical signals according to wavelength. Moreover, the structure of an optical power splitter is largely divided into a two-branch structure, a so-called Y-branch waveguide, and a multi-branch structure, a so-called star coupler.
FIG. 1 diagrammatically illustrates a Y-branch waveguide. The Y-branch waveguide includes an input waveguide 110 whose input edge receives optical signals and whose width is gradually larger from the input edge to an output side edge 115, and a first and a second output waveguides 120 and 130 that are symmetrically extended around a central line 140 from the input waveguide 110 to the output side edge 115. The Y-branch waveguide is a planar lightwave circuit and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.
The power of the combined optical signals through the input side edge of the Y-branch waveguide splits and is outputted through the first and the second output waveguides 120 and 130. It is important to make the power of the optical signals outputted from the first and the second output waveguides 120 and 130 equal, that is, to make the power split ratio uniform for the Y-branch waveguide. In addition, the power uniformization is required for optical signals of a single channel as well as optical signals of multi-channels.
FIG. 2a is a diagram illustrating mode profiles of the input waveguide 110 according to a wavelength, on the basis of the output side edge 115 of the input waveguide 110. FIG. 2b is a diagram illustrating mode profiles of the first and the second output waveguides 120 and 130 according to a wavelength on the basis of the output side edge 115 of the input waveguide 110.
Depicted in FIG. 2a are a first mode profile 210 for a first channel, and a second mode profile 220 for a second channel. The first channel has a wavelength of 1250 nm, and the second channel has a wavelength of 1650 nm. As shown in the drawing, the first mode profile 210 for a short wavelength is sharper than the second mode profile 220 for a long wavelength.
Similarly, FIG. 2b shows a third mode profile 230 for the first channel and a fourth mode profile 240 for the second channel. As illustrated in the drawing, the third mode profile 230 for a short wavelength is sharper than the fourth mode profile 240 for a long wavelength.
FIG. 3a is a diagram explaining mode inconsistency of the Y-branch waveguide for the first channel and FIG. 3b is a diagram explaining mode inconsistency of the Y-branch waveguide for the second channel.
Depicted in FIGS. 3a are the first mode profile 210 of the input waveguide 110 for the first channel and the third mode profile 230 of the first and the second output waveguides 120 and 130. As shown in the drawing, the first mode profile 210 and the third mode profile 230 are not consistent with each other. As a result, this mode inconsistency causes the output of the split optical signals to the first and the second output waveguides 120 and 130.
FIG. 3b shows the second mode profile 220 of the input waveguide 110 for the second channel and the fourth mode profile 240 of the first and the second output waveguides 120 and 130. As depicted in the drawing, the second mode profile 220 and the fourth mode profile 240 are not consistent with each other. As a result, this inconsistency causes the output of the split optical signals to the first and the second output waveguides 120 and 130.
As explained above, the outputs of the first and the second output lightwaves 120 and 130 are similar to each other. In particular, the first and the second output lightwaves 120 and 130 in the Y-branch waveguide have a bilaterally symmetrical structure around the central line 140, shown in FIG. 1. This property may be helpful to uniformize the power split ratio, however, the performance of the Y-branch waveguide is deteriorated due to output differences between the first and the second output lightwaves 120 and 130. FIG. 4 is an explanatory diagram of outputs per wavelength in the Y-branch waveguide. FIG. 4 shows output curves 250 per wavelength of the first or second output waveguide 110 or 130. As shown in FIG. 4, the output power decreases as the wavelength increases. Moreover, the variation range A also increases as the wavelength increases.
FIG. 5 is a schematic diagram of a prior art star coupler. The star coupler includes an input waveguide 310 for receiving optical signals through an input side edge, an oval-shaped slab waveguide 320 that is connected to the input waveguide 310, and the first through the fourth output waveguides 330, 340, 350 and 360 that are extended symmetrically around a central line 370 from an output side edge 325 of the slab waveguide 320. Here, the star coupler is a planar lightwave circuit, and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.
The combined optical signals through the input side edge of the input waveguide 310 are outputted through the first through the fourth output waveguides 330, 340, 350 and 360 via the slab waveguide 320. It is important to make the power split ratio uniform for the star coupler, thereby allowing the output powers of the optical signals from the first through the fourth output waveguides 330, 340, 350 and 360 to also be uniform. In addition, the power uniformization is required for optical signals of a single channel as well as optical signals of multi-channels.
FIG. 6a is a diagram illustrating mode profiles of the slab waveguide 320 according to a wavelength, based on the output side edge 325 of the slab waveguide 320. FIG. 6b is a diagram illustrating mode profiles of the first through the fourth output waveguides 330, 340, 350 and 360 according to a wavelength, based on the output side edge 325 of the slab waveguide 320.
Depicted in FIG. 6a are a first mode profile 410 for a first channel and a second mode profile 420 for a second channel. The first channel has a wavelength of 1250 nm, and the second channel has a wavelength of 1650 nm. As shown in the drawing, the first mode profile 410 for a shorter wavelength is sharper than the second mode profile 420 for a longer wavelength.
Similarly, FIG. 6b shows a third mode profile 430 for the first channel and a fourth mode profile 440 for the second channel. As illustrated in the drawing, the third mode profile 430 for a shorter wavelength is sharper than the fourth mode profile 440 for a longer wavelength.
FIG. 7a is a diagram explaining mode inconsistency of the star coupler for the first channel and FIG. 7b is a diagram explaining mode inconsistency of the star coupler for the second channel.
Depicted in FIG. 7a is the first mode profile 410 of the slab waveguide 320 for the first channel and the third mode profile 430 of the first through the fourth output waveguides 330, 340, 350 and 360. As shown in the drawing, the first mode profile 410 and the third mode profile 430 are not consistent with each other, and this mode inconsistency causes the split optical signals to be outputted to the first through the fourth output waveguides 330, 340, 350 and 360.
FIG. 7b shows the second mode profile 420 of the slab waveguide 320 for the second channel and the fourth mode profile 440 of the first through the fourth output waveguides 330, 340, 350 and 360. As depicted in the drawing, the second mode profile 420 and the fourth mode profile 440 are not consistent with each other and this inconsistency causes the split optical signals to be outputted to the first through the fourth output waveguides 330, 340, 350 and 360.
As explained above, the outputs of the first and the fourth output waveguides 330 and 360 are similar to each other, and the outputs of the second and the third output waveguides 340 and 350 are similar to each other. Further, the star coupler has a bilaterally symmetrical structure around the central line 370 shown in FIG. 5, and the first and the fourth waveguides 330 and 360 and the second and the third output waveguides 340 and 350, respectively, share similarities with each other.
Therefore, unlike the Y-branch waveguide, the star coupler has known limitations with un-uniform power split ratios and output differences between channels. This inconsistency in the power split ratio and the severe output differences between channels consequently deteriorate the performance of the star coupler.
FIG. 8 is an explanatory diagram of outputs per wavelength of the star coupler. Depicted in the drawing are a first output curve per wavelength 450 of the first or the fourth output waveguide 330 or 360, and a second output curve per wavelength 460 of the second or the third output waveguide 340 or 350. From the drawing it is shown that the output power of the first output curve per wavelength 450 tends to increase for longer wavelengths, while the output power of the second output curve per wavelength 460 tends to decrease for longer wavelengths. Moreover, the entire variation range B of the first and the second output curves per wavelength 450 and 460 is very large.